Description

Invasive hybridization, in which an introduced species may introgressively hybridize with a native taxon and threaten its persistence, is prominently featured in the conservation literature. One of the most frequently cited examples of this phenomenon involves interactions between native westslope cutthroat trout Oncorhynchus clarkii lewisi and introduced rainbow trout Oncorhynchus mykiss in a portion of the U.S. Northern Rocky Mountains (Allendorf & Leary, 1988). A recent paper by Muhlfeld et al. (2017) revisited this issue and concluded that introgressive hybridization between these taxa is ubiquitous and related primarily to climatic factors and propagule pressure from rainbow trout, findings which corroborate an earlier study (Young et al., 2016) that accurately quantified how broad riverscape gradients - not thresholds - in temperature, zoogeographic characteristics, and propagule pressure were related to levels of introgression. Yet, Muhlfeld et al. (2017) argued that there were deficiencies in that analysis and discounted that these same gradients can result in resistance of westslope cutthroat trout populations to invasive hybridization. Instead, based on extrapolation of decadal trends at nonrandom monitoring sites, they asserted that genomic extinction - the loss of all unhybridized parental forms - is inevitable for most populations of westslope cutthroat trout in the absence of physical obstacles to upstream dispersal by rainbow trout or human intervention to remove them. We question this assertion based on our field observations and critical review of the subject (McKelvey et al., 2016; Young et al., 2016), the emerging recognition of environmental constraints on introgression (Wang & Bradburd, 2014), and the ecophysiological differences between these taxa and their hybrids (Rasmussen, Robinson, Hontela, & Heath, 2012). Resolving this issue is crucial, because resources to effect conservation are limited and poorly informed efforts can be costly and have unintended consequences (Fausch, Rieman, Dunham, Young, & Peterson, 2009).